The present invention relates to driver circuits, e.g., driver integrated circuits, for driving high voltage half bridge stages, and more particularly to filter circuits used in the gate driver circuits for filtering signals to the control terminals of switches of the half bridge stages.
FIG. 1 illustrates a circuit 10 having a traditional RC filter that includes a resistor and a capacitor. A resistor R is series coupled between an inverter 12 through which input IN is received and an amplifier 14 through which an output OUT is provided. A capacitor C is coupled to a node N between the resistor R and the amplifier 14 and to the ground. Amplifiers used in such circuits have output threshold settings. For the amplifier 14, there are thresholds of VTH+ and VTH−, which are exemplary set at ⅔ and ⅓ of VCC, respectively.
A minimum pulse width of the input signal IN, i.e., near a constant time τ of the RC filter can cause a pulse width distortion between pulse widths of the input and output signals IN and OUT when the RC filter of the circuit 10 is used in the high voltage half-bridge driver.
As illustrated in FIG. 2, a pulse width of the input signal IN, e.g., 500 μs, becomes comparable with a time constant τ of the RC filter, which determines a cutoff frequency. As the voltage in the RC filter decreases from VTH+ to VTH−, an output signal OUT, for example, an illustrated 50 μs, passes through the gate driver logic circuit 10 to a control terminal of a switch in a high voltage half bridge stage.
This occurs due to a pulse distortion introduced by the RC filter and can be very dangerous, especially for a high side channel signal transmission.
As illustrated in FIGS. 3a and 3b (right side), a very short input pulse can create a situation where a “turn-OFF” signal of the high side channel is missed, and then, when a “turn-ON” signal of a low side of the same channel is generated, a shoot-through DC+/DC− current is generated in the external half-bridge. Moreover, as shown in FIGS. 3a and 3b (left side), missing the turn-ON signal of the high side channel will cause a loss of a PWM cycle.